Thermochemical conversion of biomass such as sawmill wood wastes, forestry residues and agricultural wastes into synthetic fuels is an important emerging avenue for advancement of renewable energy sources to supplement or replace fossils fuels. While air blown gasification is used for generation of lower heating value fuel gas, several variants of oxygen or steam gasification can be used for production of syngas containing minimal nitrogen. Syngas is a gas mixture containing mostly hydrogen and carbon monoxide, and is a versatile feedstock for further chemical processing into a wide range of useful fuels and chemical compounds. Syngas can be catalytically converted into methane, Fischer-Tropsch liquid fuels, methanol, dimethyl ether, or hydrogen. The methanation reaction of syngas to generate methane and byproduct water vapour is typically conducted over nickel catalysts at temperatures in the range of about 300° C. to about 400° C., and preferably at elevated pressure.
Methane is readily marketed and delivered through existing natural gas distribution infrastructure as substitute natural gas (SNG) for numerous end uses including space heating and electrical power generation. Methane has considerably higher energy density than hydrogen, and can be converted into syngas or hydrogen by catalytic steam reforming. Modern combined cycle power plants are conveniently fueled by natural gas. Methane is also a particularly advantageous fuel for future high temperature fuel cell power plants using highly endothermic internal steam reforming of natural gas to recover high grade heat generated by the fuel cell stack.
The reaction of steam with biomass to generate syngas is highly endothermic, hence conducted with direct or indirect heating by partial oxidation with air or oxygen; and is typically conducted at much higher temperature than the subsequent exothermic methanation reaction. The thermal mismatch between gasification and methanation reactions is detrimental to process efficiency.
Hydrogasification has previously been investigated for gasification of biomass. The key reaction is hydrogenation of carbon to form methane, whose exothermicity is a great advantage compared to other gasification approaches. As hydrogen is a premium fuel, its consumption in large amounts has presented the appearance of a major economic barrier.
The endothermic nature of the syngas formation reaction from the reaction of biomass pyrolysis gas and steam requires enthalpy heat to be added (typically by partial combustion with added oxygen). Temperatures well in excess of 650° C. are typically required to reduce tars to reasonable levels.
The gas composition produced in biomass gasification approaches a complex equilibrium established between CO, CO2, H2, H2O and CH4 which is a function of temperature, pressure and overall gas composition. Reforming reactions producing syngas increasingly dominate the equilibrium at temperatures above 650° C. at the expense of hydrocarbons, CO2 and water.
The use of catalysts, such as the use of olivine, dolomite or nickel coated media in fluidized beds, to enhance the rate of syngas formation is well known. These catalysts allow a faster reaction towards syngas equilibrium favoured under the process conditions. Catalysts have also been used in a secondary bed in series with the gasifier for the reduction of tars contained in the syngas or producer gas.
An oxygen blown entrained flow gasifier may typically operate at about 1300° C. to 1500° C., at which temperatures methane and higher hydrocarbons are all nearly entirely converted to syngas. This has the important advantage of almost completely eliminating tar constituents, but the disadvantage for SNG production that all of the product methane must be generated by the exothermic methanation of syngas at much lower temperature than the gasification temperature.
Indirect steam gasifiers (such as the US Battelle/FERCO “Silvagas” system, the Austrian fast internally circulating fluidized bed (FICFB) system, and the Dutch ECN “Milena” system) operate at about 850° C. These systems use twin bed configurations, in which fluidized granular heat transfer media is circulated between a gasification zone in which steam reacts with the biomass to produce syngas and char, and an air-blown regeneration zone in which the char is combusted to reheat the media. The product syngas contains a significant admixture of methane generated within the gasifier. While downstream processing is required to convert or remove tar constituents, an important advantage for SNG production is that only about 55% to 60% of the final product methane must be generated by methanation of syngas, since a useful fraction of the methane was already produced with the syngas.
Some recent improvements to the twin bed gasification approach have been based on adsorption enhanced reforming (“AER”) in which a CO2 acceptor such as lime or calcined dolomite is included in the granular media to remove carbon dioxide by carbonation from the gasification zone operating typically at about 600° C., and to release the carbon dioxide by calcining in the regeneration zone operating typically at about 800° C. The AER process has been disclosed by Specht et al. (European patent publications EP 1,218,290 B1 and EP 1,637,574 A1). The principle of the AER process is to generate hydrogen-rich syngas by shifting the reaction equilibria of the steam reforming and water gas shift reactions by CO2 removal. The AER process has been tested in the FICFB twin bed system, and is being developed for SNG production by using a molten salt methanation reactor to convert the syngas into methane.
Twin bed indirect steam biomass gasifiers, and experimental AER systems derived from twin bed gasifiers, have been operated at atmospheric pressure. Air blown combustion regeneration of pressurized fluidized beds would present challenges. ECN have considered operation of the Milena twin bed gasification system pressurized to about 7 bara.
There is a need to provide more efficient internally self-sustaining generation of the hydrogen needed for hydrogasification, which otherwise is an extremely attractive approach for conversion of biomass and other carbonaceous feedstocks into methane and other high value synthetic fuels.